1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker""s fabric of the on-machine-seamable variety, such as an on-machine-seamable press fabric for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the fibers in the web to one another to turn the fibrous web into a sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The web, or newly formed paper sheet, itself is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the web to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a paper machine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
In brief, the seam region of any workable on-machine-seamable, or OMS(copyright), press fabric must behave under load, that is, under compression in the press nip or nips, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the paper product being manufactured from being marked by the seam region. OMS is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable press fabric, because of the comparative ease and safety with which it could be installed on a press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. Then seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less timeconsuming to install an on-machine-seamable press fabric, than it is to install an endless press fabric, on a paper machine.
There are several methods for producing a press fabric that can be joined into endless form on the paper machine with such a seam. One method is to flat-weave the fabric, in which case the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric.
Originally, single monofilament strands were used in both the machine and cross-machine directions of on-machine-seamable press fabrics. The relative stiffness of monofilament ensures that it will have the requisite good seaming-loop formation properties. Experience showed, however, that single monofilament strands are difficult to weave and have insufficient elasticity in the machine direction for many kinds of contemporary presses. Tensile failure and seam breakage were frequently observed.
Another difficulty is presented by the very open, rigid, incompressible structure of base fabrics woven from single monofilament. For some papermaking applications, this incompressibility is not a problem, and may even be ideal. However, for positions that have poor auxiliary fabric dewatering capacity, or produce mark-sensitive paper grades, a softer, more compressible base fabric is needed.
A more compressible base fabric may be obtained by weaving with multifilament or plied monofilament yarns, instead of with single monofilament strands. However, yarns of these types do not have the rigidity necessary for good loop formation or for maintaining the integrity of the seam area during loop meshing when the seam is to be closed. Moreover, because yarns of these types are twisted, loops formed from them tend to rotate about axes lying in the planes of the loops. When this rotation, known as the secondary helix effect, occurs, it causes the loops to depart from the ideal orientation needed to form the seam, that orientation being such that the planes of the loops are parallel to one another, align with the machine direction, and are perpendicular to the plane of the base fabric, and that the loops themselves align widthwise across the base fabric. Departure from this ideal orientation makes it difficult, if not impossible, to interdigitate the loops at each end of the press fabric properly during closure, as well as to direct a pintle through the passage defined by the interdigitated loops.
Various attempts have been made in the prior art to overcome these difficulties by making the loop-forming MD yarns act like monofilament, although, as will become apparent below, loops formed by monofilament yarns are not necessarily free of orientation and alignment problems. In U.S. Pat. No. 5,005,610, the MD yarns in an on-machine-seamable papermaker""s fabric have a composite structure including braided monofilament strands. The braided yarn forms seaming loops which resist deformation and, because they are balanced with regard to twist, form seaming loops which are not susceptible to xe2x80x9csecondary helix effectxe2x80x9d rotation from a preferred orientation.
In U.S. Pat. No. 5,204,150, the MD yarns in an on-machine-seamable papermaker""s fabric are plied/twisted yarns extruded from a resin which partially melts during the heat-setting of the fabric, giving the MD yarns a monofilament-like character. Even though not balanced due to the twisting and plying, the fusion caused by the partial melting of the individual ends prevents loop rotation from a preferred orientation.
Finally, in U.S. Pat. No. 5,391,419, the MD yarns of an on-machine-seamable papermaker""s fabric are plied/twisted yarns having a coating which gives them yarn a monofilament-like structure. The coating may be either permanent, semi-permanent or soluble. Even though the yarns may not be balanced, the coating prevents loop rotation.
Another approach toward improved seaming loop uniformity and stability, useful no matter what the form of the MD yarns, is shown in U.S. Pat. No. 5,913,339. This patent shows an on-machine-seamable papermaker""s fabric having first and second layers of machine-direction (MD) yarns interwoven with a plurality of cross-machine-direction (CD) yarns. The fabric is woven in a modified endless weave in which seaming loops are formed by the MD yarns when they alternate between the first and second layers at the ends of the fabric. Additional CD yarns are interwoven with both MD layers at each end of the fabric between the last CD yarn and the seaming loops in a balanced weave which establishes vertical and horizontal alignment for the seaming loops. More specifically, the additional CD yarns correct for any misalignment of the seaming loops arising from the pattern in which the fabric is woven, rather than from the character of the yarns themselves.
While it is important to maintain the uniformity, alignment and proper orientation of the seaming loops, a more important technical challenge with on-machine-seamable press fabrics is presented by desirability of providing the seam and regions immediately adjacent thereto with permeability and compressibility characteristics substantially identical to those of the rest of the press fabric, or, in other words, the body of the press fabric. In connection with this technical challenge, which relates to the minimization of any marking of the paper sheet by the seam region, it should be recalled that the manufacture of an on-machine-seamable press fabric includes the attachment of a staple fiber batt to one or both, sides thereof. The attachment may be effected by a process called needling (fiber locking) or by hydroentangling, while the on-machine-seamable base fabric is in endless form. Once the desired amount of staple fiber batt has been attached, the loop-forming pin or pintle is removed to place the press fabric into flat, or open, form for shipment and eventual installation on a paper machine. At that time, the staple fiber batt must be cut in the vicinity of the seam to completely separate the two ends of the press fabric from one another. Typically, the staple fiber batt is cut in a manner that enables it to form a flap over the seaming loops when the press fabric is rejoined into endless form during installation on a paper machine. For this reason, the two ends of the press fabric are often referred to as the xe2x80x9cflapxe2x80x9d end, which has the flap of staple fiber material extending over and beyond the seaming loops, and the xe2x80x9cno-flapxe2x80x9d end, which has a space, adjacent to its seaming loops, into which the flap on the other end fits when the fabric is joined into endless form. It should be noted that, when the fabric is installed on a paper machine, its orientation is such that the xe2x80x9cflapxe2x80x9d end will lead the xe2x80x9cno-flapxe2x80x9d end through the press nip or nips to prevent the flap from wearing away too quickly.
On the other side, the so-called xe2x80x9crollxe2x80x9d side, of the press fabric, some staple fiber batt may be removed from the seaming loops to facilitate subsequent passage of a pintle therethrough. The removal of this generally small amount of staple fiber batt makes the seam region slightly more permeable to air and water than the body of the press fabric. This difference in permeability or flow resistance, perhaps ever so slight, is sufficient to cause sheet marking in some situations.
Several approaches toward solving this problem have heretofore been taken. One approach involves the use of stuffer yarns with the pintle when the press fabric is being joined into endless form on the paper machine. In another approach, a press fabric comprises two on-machine-seamable base fabrics, one fitting inside the endless loop formed by the other, the two base fabrics being laminated to one another during the needling process. The seam regions of the inner and outer base fabrics are offset slightly with respect to one another, so that the seam region of each will coincide with a non-seam region of the other. Once the desired amount of staple fiber batt has been attached to the inner and/or outer surfaces of the laminated base fabrics, the loop-forming pin or pintle of each on-machine-seamable base fabric is removed to place the on-machine-seamable press fabric into flat form for shipment and eventual installation on a paper machine. At that time, the staple fiber batt must be cut in the vicinity of the seam in the outer of the two on-machine-seamable base fabrics to completely separate the two ends of the press fabric from one another. As above, the staple fiber batt may be cut in a manner that enables it to form a flap over the seaming loops when the press fabric is rejoined into endless form. Some of the staple fiber batt may also be removed from the seaming loops of both the inner and outer on-machine-seamable base fabrics to facilitate the subsequent passage of pintles therethrough.
In yet another approach, disclosed in U.S. Pat. Nos. 5,476,123 and 5,531,251 to Rydin, one or more extra CD yarns are woven with the seaming loops of at least one end of a base fabric of an on-machine-seamable press fabric. The extra yarn or yarns are woven only with those portions of the seaming loops that are on one side of the fabric, that side preferably being the paper-supporting side. The extra CD yarn or yarns form an extension of the CD yarn system of the base fabric at the seaming loop or loops, conforming the seam region more closely to the rest of the base fabric, so that staple fiber batt will be better anchored to the seam region and so that the possibility of sheet marking by the seam region will be minimized.
The present invention provides another approach toward providing the seam region of an on-machine-seamable press fabric with permeability and compressibility characteristics substantially identical to those of the body of the press fabric in order to minimize the marking of a paper sheet by the seam region. It also may serve as a means for maintaining the uniformity, alignment and proper orientation of the seaming loops.
Accordingly, the objective of the present invention is to provide an on-machine-seamable papermaker""s fabric with seam regions having compressibility and permeability characteristics like those of the body of the fabric. This objective is met with the present on-machine-seamable papermaker""s fabric, which is preferably woven in a modified endless weaving technique from a system of machine-direction (MD) yarns and a system of first cross-machine-direction (CD) yarns. The papermaker""s fabric has a rectangular shape with a length, a width, two lengthwise edges and two widthwise edges. The yarns of the system of MD yarns are interwoven with the yarns of the system of first CD yarns in a first weave pattern, and form the body of the fabric.
The MD yarns extend back and forth continuously for the length of the papermaker""s fabric between the two widthwise edges, at each widthwise edge forming a plurality of seaming loops.
The papermaker""s fabric also includes a system of second CD yarns, which are interwoven with the yarns of the system of MD yarns in a first region along one of the two widthwise edges of the papermaker""s fabric between the system of first CD yarns and the seaming loops. The second CD yarns are woven with the MD yarns in a second weave pattern which may be the same as or different from the first weave pattern. The first region includes more than two second CD yarns, and differs from the body by at least one of the following:
a) the denier of at least some of the second CD yarns is different from the denier of the first CD yarns;
b) the spacing between at least some of the second CD yarns is different from the spacing between the first CD yarns; and
c) the second weave pattern is different from the first weave pattern.
The papermaker""s fabric further includes a system of third CD yarns, which are interwoven with the yarns of the system of MD yarns in a second region along the other of the two widthwise edges of the papermaker""s fabric between the system of first CD yarns and the seaming loops. The third CD yarns are woven with the MD yarns in a third weave pattern which may be the same as or different from the first weave pattern. The second region includes more than two third CD yarns, and differs from the body by at least one of the following:
a) the denier of at least some of the third CD yarns is different from the denier of the first CD yarns;
b) the spacing between at least some of the third CD yarns is different from the spacing between the first CD yarns; and
c) the third weave pattern is different from the first weave pattern.
The present invention will now be described in more complete detail, with reference being made to the figures identified below.